I am looking for the input electrical specifications. I can't find it anywhere. I did read that two GPIO pins are rated at 3.3 volts but nowhere did it say which pins.

I have a system that has a GPS and an Edison. All I need to do is take the GPS (3.3 V) Tx pin and connect it to the Edison UART Rx pin. A level shifter breakout board will work but I would assume so would a resistive divider. Ideally a single resistor working against the UNSPECIFIED input resistance. Where is the electrical specification and has anyone used just a series resistor to go from a 3.3V to the UART Rx pin?

Could you tell me where did you read this? There’s a section in the Edison Module Hardware Guide (section 3.5) that talks about the PMIC and its available GPIOs. Is this the information you are referring to?

Yes. "Seven general purpose 1.8 V I/Os, with two of them supporting up to 3.3 V"

Perhaps that is not referring to the Edison GPIO but no matter what, I still am looking for what the Edison GPIO input looks like. Specifically, what do I need to do to drive the input from a 3.3V source? Maybe I can use a voltage divider or just a series resistor, or maybe not. The point is, I need an electrical specification for the input to know what I am driving and I can't find it.

You can take a look at Table 27 and Table 28 of the Edison Module Hardware Guide to check the specifications of the GPIO pins. Regarding the two GPIO pins that support 3.3V, these belong to the PMIC, and the only visible pin of the PMIC is Vsys so this is not an option.

For suggestions on how to translate the 3.3V to 1.8V I would suggest you to check the following threads were this is discussed.

In terms of level shifting, I have been using the Sparkfun or Adafruit bidirectional ones and while they work fine, I was looking for something simple for the case where I only needed to drop a single pin from 3.3 volts to 1.8 and from the HG I have enough specs to figure it out.